The following abbreviations are herewith defined, at least some of which are referred to in the ensuing description of the prior art and the present invention.    AAA Accounting, Authorization and Authentication    DHCP Dynamic Host Configuration Protocol    HTTP Hyper Text Transfer Protocol    IM Instant Messaging    IP Internet Protocol    PDA Personal Digital Assistant    QoS Quality of Service    SIMPLE SIP for IM and Presence Leveraging Extensions    SIP Session Initiation Protocol    VPN Virtual Private Network    XCAP XML Configuration Access Protocol
A main function of a presence-based SIP/SIMPLE communications network is to improve communications between a presence watcher (or, caller) and a presentity (or, callee). In such a system, the communication efficiency for a specific presence watcher in contacting a specific presentity depends on how much the presence watcher knows about the real-time status of the presentity. For example, the presence watcher may like to know the following information prior to contacting the presentity:                Presence Information:                    Availability: Is the presentity available to be contacted?            Activity: What is the current activity of the presentity?            Local Time: Is this the right time to contact to the presentity?            Devices/Media Types: How I can reach the presentity?            More . . .                        Communication Preferences:                    What is the presentity's preferred language?            What is the presentity's preferred device?            What is the presentity's preferred security mode/protocol?                        
The real-time presence information can be used by the presence watcher to make an informed decision as to whether or not the presentity is currently available to take part in an interaction (e.g. phone call, IM). This all normally works well when the presentity happens to be located within their home network. But, if the presentity moves/roams from their home network to another SIP/SIMPLE network (e.g., visited network) then there is no effective way today for enabling a presence watcher to obtain the presentity's presence information or to send the presentity an IM message. This is because there is no effective way today for enabling a presentity to maintain their presence/IM service when they move/roam from their home network to a visited network. This problem and other problems are solved by the present invention.